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What is the max CBR if you add fly ash to the soil?

What is the max CBR if you add fly ash to the soil?

I just got a CBR of 125 with 12% fly ash added to the soil.  Is that even possible?

LL = 50; PL = 12; PI = 38.  USCS = CL/CH.  We added the fly ash, waited 10 minutes, compacted to target density, waited 2 hours, soaked 4 days and tested.  

I was trying to determine the design CBR for when the project gets going.  The client wanted 20-25, and estimated we would need to add 15-18% fly ash to get that.

Thanks in advance


RE: What is the max CBR if you add fly ash to the soil?

Yes, it's possible. Fly ash has cementitious properties.  The reaction is usually slow, but in the presence of certain soil chemistry it will react and hydrate to the properties you've noted.

RE: What is the max CBR if you add fly ash to the soil?

The problem you will have is getting the fly ash thoroughly mixed.  It is one thing to do it in the lab, quite another to mix it with the clay in the field.  Can be done, but takes a lot of effort, supervision, and testing.

RE: What is the max CBR if you add fly ash to the soil?

Ron, thanks for the post, I just didn't expect this much change this fast.  Our typical CBRs in the area are around 3

hokie66, I agree with the constant supervision required in the field.  I was just shocked at the in-lab value.  

More info about the material:  the proctors were done per ASTM D1557 "Modified"; we ran the proctor with fly ash according to D558 but compacted it in 5 layers with the bigger rammer (keep it modified per project specs);  the CBR was then compacted to 90% MDD with %moisture at 5% over optimum (close to field conditions)

Most material I have read states to expect a maximum CBR of around 30.  So, 125 seems to have surpassed that just slightly.

RE: What is the max CBR if you add fly ash to the soil?

Make sure you don't have sulfur compounds in the soil and use lime (CaO) with caution when blending with flyash. There are damaging case histories with high pH, sulfide and flyash.  Huge swell and high swell pressure.  Such remineralization can take weeks (months) and not show up on the 48-hour CBR soaking period (i.e., when you measure swell).


¡papá gordo ain't no madre flaca!

RE: What is the max CBR if you add fly ash to the soil?

Will excessive fly ash result in shrinkage cracks the way using too much Portland cement will?

     Maybe the tyranny of Murphy is the penalty for hubris.  -

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